1. Field of the Invention
This invention relates to scanning tunneling microscopy systems, and, more particularly, to tips used in scanning tunneling microscopy systems.
2. Description of the Related Art
Scanning tunneling microscopy ("STM") systems were developed to use electron tunneling for structural and spectroscopic imaging of the surfaces of samples. In practice, electron tunneling occurs when a voltage is placed across two conductors separated by a sufficiently thin insulating layer. In STM systems, the first conductor is the tip of the STM system and the second conductor is the sample whose surface is to be imaged. The insulating layer may be a liquid layer or a vacuum between the STM tip and surface of the sample. The current between the conductors (resulting from the voltage across them and the insulating layer) is a function of the conductor or electrode separation and the nature of the electronic states of the tip and sample.
To perform structural imaging in STM systems, the tunneling current between the tip and sample is measured while scanning the tip across the surface of the sample in, for example, a raster pattern. If the distance between the sample and the tip is adjusted to keep the current constant, a plot of the tip-sample distance versus position of the tip provides an indication of the structure of the surface of the sample. To perform spectroscopic imaging, the position of the tip over the sample is fixed for a period of time while the voltage across the tip and sample is varied. A plot of the deviation of the tunneling current vs. voltage for different positions of the tip enables the spectroscopic imaging of the surface of the sample.
In each of these imaging systems, only the total current of the tunneling electrons is measured in the process of imaging the surface of the sample. The energy or velocity, direction of velocity, and spin orientation of tunneling electrons is not determined. This information, however, could be used to provide higher resolution imaging of the surface or properties of the surface of the sample. Determination of these properties is difficult with conventional STM systems because the tunneling electrons typically scatter after entering the sample. In order to attempt to determine one or more of these properties of tunneling electrons, the simple sharpened metal tips of conventional STM systems have been modified. For example, Packard et al., Europhysics Letters 26, 97 (1994) and Nunes and Amer, Appl. Phys. Lett 63, 1851 (1993), teach the use of semiconductor tips with a narrowly populated band structure to improve spectroscopic resolution.
Johnson and Clarke, J. Appl. Phys 67, 6141 (1990), suggest the use of magnetic tips to accept tuneling electrons with selected spin orientation to enable magnetic imaging of samples. These tips are formed from a single component with material properties carefully selected. Gutierrez et al U.S. Pat. No. 4,985,627 described another STM magnetic imaging technique involving optical excitation of polarized electrons in the tip. Implementing either of these methods is difficult because they require spin discrimination or spin selection in the tip itself. Structured tips for improved spectroscope imaging have also been suggested. Dykhne et al., Physics Doklady 41, 233 (1996) and Hauser et al., Superlattices and Microstructures 20. 623 (1996) suggest the use of a quantum dot tip. U.S. Pat. No. 5,581,083, suggests the use of a Schottky barrier tip. E. L. Courtens et al, IBM Technical Disclosure Bulletin Vol. 35, No. 3 describe the use of a hollow tip with a thin membrane. Each of these suggested structured tips, however, still only measures the total tunneling current, which is a function of the potential at the tip. In addition, with these tips energy spectra can not be measured without changing the sample-tip bias. The need exists for a tip structure which permits measurement of the tunneling electron energy and/or angular distribution independent of the sample bias. W. J. Kaiser and D. Bell, U.S. Pat. No. 4,823,004 describe a STM geometry employing an ultra-thin sample which transmits tunneling electrons, allowing the angle and energy of the transmitted electrons to be measured. However this method is not generally applicable, requiring the use of very specially prepared samples. In addition, a need exists for tip which enables higher resolution magnetic imaging of the surface or surface properties of a sample, i.e., imaging in the nanometer or sub-nanometer range by a method in which the electron spin discrimination is performed remotely from the tip.